All clinical trials of anti-infective agents have employed microbiologic tests to assess the in vivo activity of the chemotherapeutic agent under study. Similar tests are needed for clinical trails of antiretrovirals. It is the goal of the Virology Core Laboratory of the UCSF ACTU to perform tests known or suspected of measuring in vivo antiretroviral effect in a reproducible, perform tests known or suspected of measuring in vivo antiretroviral effect in a reproducible, rapid and accurate fashion. Currently the tests most widely used for assessing in vivo antiretroviral effect are the serum HIV p24 (core) antigen, and culture of peripheral blood mononuclear cells for HIV. Although changes in the serum concentration of p24 antigen appear to be a good measure of antiretroviral effect with reverse transcriptase inhibitors, simple culture of leukocytes for HIV has not. The ACTG is therefore moving toward quantitative culture of plasma as another marker of in vivo antiretroviral effect, and other tests (e.g. measurement of HIV cDNA or genomic RNA after PCR amplification) are being considered. The UCSF ACTU Core Virology Laboratory has been performing HIV culture and p24 determination since 1987, and has been certified continuously for p24 antigen testing since 1989 and for HIV culture since 1990. In addition, we have experience in some as-yet uncertified procedures such as quantitative microculture of cells and plasma, and PCR quantitation of HIV cDNA and genomic RNA. The specific aims of the UCSF ACTU Core Virology Laboratory are to perform HIV cultures, p24 antigen quantitation, and other virologic testing as needed for ACTU's for later testing; to maintain our proficiency by participating in ACTG quality assurance programs, and to work with the ACTG nationally to develop and validate other markers of in vivo antiretroviral effects.